Mode-locked lasers, for example, mode-locked fiber lasers, generate relatively short pulses of laser light. To achieve the generation of such pulses, many methods are used in the prior art. For example, pulses may be generated by inserting in the laser cavity a saturable absorber. The saturable absorber is a material that preferentially transmits light having high intensity. Therefore, in these embodiments, any random fluctuation in the intensity of the light propagating within the laser cavity is preferentially selected. Since these lasers include highly reflective mirrors at both ends thereof, even relatively inefficient intensity selection will result in the formation of pulses as light travels back and forth in the laser cavity. A disadvantage of these mode-locked lasers is that they are typically not tunable in wavelength.
In another type of mode-locked lasers, the saturable absorber is replaced by an optical component that periodically changes its absorption coefficient at the lasing frequency. For example, this may be achieved by using acousto-optic modulators, or electro-optic modulators. By selecting the frequency at which the absorption coefficient is modulated, it is possible to select for light traveling within the lasing cavity in pulses having a round trip time around the cavity that is a multiple of the frequency at which the absorption coefficient is modulated. Therefore, only a few or one pulse is selected within the cavity, which produces the pulsed output. Conventional mode-locked lasers using this technology are also typically not adjustable in wavelength.
To produce tunable mode-locked lasers, Sorin et al., in U.S. Pat. No. 6,091,744 issued on Jul. 18, 2000, have proposed using a laser in which one end thereof includes a plurality of fiber Bragg gratings longitudinally spaced apart from each other, each fiber Bragg grating being reflective at a respective wavelength. A tunable filter is inserted in the laser cavity. By selecting a specific wavelength using the tunable filter, reflection occurs at a specific one of the fiber Bragg gratings, which provides selection of wavelength in discrete steps. Such lasers have been shown to be advantageous in telecommunication applications in which specific wavelengths are used to transmit information over different channels. However, these set-ups require the use of a tunable filter and are therefore relatively expensive. Furthermore, tunable filters are typically relatively fragile components and, therefore, the resulting lasers are not very robust. Yet, furthermore, this laser is not continuously adjustable in wavelength, which may prove a disadvantage in many applications.
PCT Publication Ser. No. WO 03/043149 published on May 22, 2003 by Duguay et al. describes an electronically tunable laser using wavelength selective reflectors. In this tunable laser, a gain fiber is coupled at both ends thereof to optical fibers in which paired sets of fiber Bragg gratings are formed, the fiber Bragg gratings being reflective at different wavelengths and each of the fiber Bragg grating in each pair being located in a respective one of the optical fibers. The fiber Bragg gratings in each pair are all distanced from each other by a substantially similar distance and are longitudinally offset from each other. Therefore, a round trip time inside the cavity does not depend on the specific pair of fiber Bragg gratings that reflects each wavelength. An optical modulator is inserted between one of the optical fibers containing the fiber Bragg gratings and the gain medium so as to select times at which pulses are permitted to travel inside the laser. By properly selecting the delay between two successive moments at which pulses are allowed to pass through the modulator, pulses reflected by a specific pair of fiber Bragg gratings are preferentially selected in the tunable laser. Consequently, the wavelength of the laser light produced by the laser is selected using the optical modulator by selecting the pair of fiber Bragg gratings that is used to reflect the light. However, this arrangement is relatively complex and requires relatively precise timing of the modulator to operate properly.
Against this background, there exists a need in the industry to provide an improved mode-locked laser. An object of the present invention is therefore to provide such a device.